In the railway rolling stock art, it has been common practice to support the opposed ends of a freight car body on spaced-apart wheeled truck assemblies for travel along a railway track. The standard truck assembly is commonly referred to as a three-piece truck because its priciple structural members are a pair of laterally spaced side frames which extend longitudinally of the freight car body, and an elongated bolster which extends transversely of the freight car body. The side frames of such a truck typically are supported by a pair of wheel and axle sets which are spaced apart along the track. The longitudinal ends of the bolster are received in openings or windows in the opposed side frames, respectively, and are supported therein by respective spring sets to permit movement of the bolster relative to the side frames. Each spring set typically includes plural elongated coil springs which extend between a spring seat portion the side frame window and a respective undersurface of the bolster end spaced above the side frame spring seat. The freight car body is supported adjacent its longitudinal ends on respective centerplate portions of the truck bolsters.
Normal railway track conditions frequently includes rail running surface variations resulting from such causes as differential track settling due to non-uniform ballast or foundation under the railway ties, excessive rail wear and/or rail misalignment, and rail joints. Under normal operating conditions, these and other non-uniformities can result in truck wheel movements which impart sufficient energy to the truck suspension system to cause the car body to rock, bounce, or sway. If track variations encountered in operation are such that the truck wheel movements become coupled, through the truck spring suspension, to the car body motion, the dynamic response of the system can be reinforced and amplified and the truck wheels can become unloaded. More specifically, car body rocking motion of sufficient magnitude will compress the load springs alternately at the opposed ends of the bolster to a solid or near solid condition. The response of the load springs as they alternately compress and extend can reinforce and amplify the car body rocking motion. As a result, the forces between the wheels and the rail can be significantly reduced alternately on the laterally opposed sides of the truck. In the extreme the wheels will lift from the rail. Any such wheel unloading substantially increases the risk of a derailment.
The most common expedient presently employed to control rocking and other dynamic responses of railway car bodies and trucks is the friction assemblies which provide bolster-to-side frame damping and fit up. Such friction assemblies commonly include rigid metallic friction wedges or shoes that are carried in bolster pockets and are maintained in frictional engagement with respective side frame column wear surfaces. The friction shoes dissipate suspension system energy by frictionally damping relative motion, especially vertical motion, between the bolster and the side frames. Such rigid friction shoes have been used in practically all freight car trucks built in the past 40 years.
More recently, elastomeric friction elements have been developed for use in place of rigid friction shoes. For example, I have previously developed elastomeric friction elements as shown and described in U.S. Pat. No. 4,230,047, now U.S. Reissue Pat. No. 31,784, and U.S. Pat. No. 4,295,429, now U.S. Reissue Pat. No. 31,988. The elastomeric friction elements disclosed in these patents offer improved damping for all modes of relative bolster to side frame motion. I have also developed railway truck bolster friction assemblies that include combined rigid and elastomeric elements where the rigid element is maintained in frictional engagement with a side frame column wear surface and a resilient, deformable elastomeric element is disposed between the rigid friction element and a sloping inner surface of the bolster pocket.
Also known in the art are so-called winged friction shoes which include laterally projecting wing portions with sloping engagement surfaces disposed for frictional engagement with complementary sloping inner surface portions of a bolster pocket. Such winged friction shoe arrangements have been contemplated heretofore only for constant or fixed bias frictional damping structures in which the retention spring that maintains the friction shoe in biased engagement with the bolster pocket surface and the side frame column wear surface is supported with respect to the friction shoe by a spring base or seat portion of the bolster that extends beneath the friction shoe. Such arrangements are characterized as offering fixed or constant bias because the compression of the friction assembly retention spring remains essentially unchanged during relative movement, especially vertical movement, of the bolster with respect to the side frame. Accordingly, in a constant bias arrangement the biasing force of the retention spring upon the friction shoe, and thus the frictional force sustained between the friction shoe and the respective confining surfaces of the column wear plate and the bolster pocket inner surface, remains essentially constant throughout relative motion between the bolster and the side frames of the truck and for all freight car lading conditions from empty to fully loaded.
Fixed bias frictional damping arrangements, as above characterized, are distinguished from variable bias arrangements wherein the friction assembly retention spring extends between the friction shoe and the side frame spring seat, and the frictional response of the friction shoe thus varies with variation in the compression of the friction assembly retention spring. Accordingly, in variable bias arrangements the compression of the friction shoe retention springs, and therefore the magnitude of friction between the friction shoe and the side frame column, varies as the bolster moves vertically with respect to the side frame such as occurs under normal operating conditions or when an empty car is being loaded with freight. Thus, with a variable bias friction assembly, the frictional damping response of the friction assembly is different for loaded and empty cars whereas with a fixed bias friction assembly the damping response remains essentially uniform for all lading conditions.
For fixed friction arrangements as above characterized, the friction shoe typically includes an elongated spring pocket extending therewithin and usually disposed laterally between a pair of wing portions which include laterally spaced bolster contacting surfaces as above described. The spring pocket accommodates an optimized retention spring of sufficient length and coil diameter to provide adequate frictional damping, especially for empty or lightly loaded conditions. In known variable friction arrangements, the available vertical distance between the friction shoe and the side frame spring seat generally has been considered to be more than sufficient, even under fully loaded conditions, to accommodate a spring with a suitable design characteristic in accordance with higher capacity loaded car standards.
Among the issued patents known to me which pertain to railway truck bolster friction apparatus of the type to which my present invention relates, the following may be material to my invention as characterizing the state of the art: U.S. Pat. Nos. 3,977,332, 4,109,585, 4,274,340, 3,802,353, 4,570,544, 4,256,041, 4,254,713, 2,456,635, 2,465,763, 2,512,829, 2,574,348, 3,712,247, 2,603,166, 3,109,387, 2,548,223 and 3,072,076.